Thermoreflectance

Thermoreflectance is a method for determining the thermal diffusivity and thermal conductivity of thin films with thicknesses in the nanometer range. In contrast with the conventional laser flash method, there is no infrared detector used to measure the temperature increase in the sample following a short laser pulse. Instead, the temperature-dependent reflectivity of a surface is used to generate the measurement signal (voltage change).

The thin film is heated by a short laser pulse (pump laser). At the same time, an additional laser (probe laser) is left on continuously. The laser light of the probe laser is reflected by the film surface to the detector. The absolute value of voltage change in the detector is proportional to the temperature change of the film’s surface. A model calculation on the basis of the voltage change (thermogram) yields the thermal diffusion time and thermal diffusivity of thin films.

The thermal diffusion time (t) is dependent on the thickness (d) and thermal diffusivity (a). The possible thermal diffusion time ranges can be seen in figure 1. The lower limit for the LFA 467, for example, is ~500 µs which is comparable to a copper plate with a thickness of 200 µm. In contrast with this, the PicoTR (pico-second thermoreflectance apparatus) is able to measure a molybdenum film with a thickness of 100µm. For applications in the range between the LFA and PicoTR, the more cost-effective NanoTR (nano-second thermoreflectance apparatus) is available.

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